John Podesta, outgoing aide to President Obama, expressed his regret this week for not having been able to secure disclosure of secret UFO documents withheld by the U.S. government.

“Finally, my biggest failure of 2014: Once again not securing the #disclosure of the UFO files. #thetruthisstilloutthere,” Podesta tweeted on his last day working at the White House.

Podesta, who was President Clinton's Chief of Staff, is expected to join Hillary Clinton's presidential campaign team.

Podesta was a big fan of the TV show "X-Files" and has long been a proponent of UFO disclosure.

Presidential advisor John Podesta

In 2002, Podesta promoted the idea of opening the government’s files on UFOs when he spoke at a meeting of the Coalition for Freedom of Information.

“I think it’s time to open the book on questions that have remained in the dark, on the question of government investigations of UFOs,” said Podesta. “We ought to do it, really, because it’s right. We ought to do it because the American people, quite frankly, can handle the truth. And we ought to do it because it’s a law.”